In general, magnetic sensors having hall elements are for use in proximity sensors, linear position sensors, rotation angle sensors, and the like, as sensors that detect positional information of a magnet. In addition to them, magnetic sensors having hall elements are widely used as current sensors each that measure an amount of current flowing across a current conductor in a non-contact state, by detecting the magnetic field induced by the current flowing across the current conductor.
Further, since the hall elements have a magnetoelectric conversion function that generates a hall electromotive force signal depending on the magnitude of the magnetic field that has been input, they are widely used as magnetic sensors. The hall elements, however, have an offset voltage (unbalanced voltage) which means that a limited voltage, which is not zero, will be output, even if there is no magnetic field, which is even in a non-magnetic field state.
Hence, as to a magnetic sensor having the hall element, there are hall element driving methods generally known as Spinning current method or Connection communication method, for the purpose of cancelling the offset voltage of the hall element. In such a method, the positions of a terminal pair for causing the drive current to flow across the hall element and the positions of another terminal pair for detecting the hall electromotive force signal are switched periodically according to a clock called chopper clock.
The above Spinning current method for the purpose of cancelling the offset voltage can be configured with a switch circuit also in a CMOS semiconductor circuit. Thus, a hall electromotive force detection circuit for achieving a magnetic sensor with high accuracy is generally provided with the switch circuit for achieving the Spinning current method.
In a magnetism detection device that utilizes a hall sensor, since the hall element has the offset voltage, the offset voltage is removed. As an example, Spinning current principle described in Non-Patent Document 1 is known.
In such a magnetism detection device, however, when the polarities of the input voltages to a differential amplifier become different by switch operation of the switch, the settling time of the differential amplifier is finite, and in addition, the settling time at the rising of the input voltage and the settling time at the falling thereof are different. This will bring an influence on voltage waveforms of the output voltage of the differential amplifier and the output voltage of the switch. Since the voltage waveforms of the output voltage of the switch are different as described above, even if an integrator integrates the output voltage, there is a problem in that neither the offset voltage of the hall element nor the offset voltage specific to the differential amplifier can be removed.
Accordingly, in order to address the drawback of this type, for example, Patent Document 1 describes a magnetism detection device capable of removing the offset voltage of the hall element and the offset voltage specific to the amplifier that amplifies the output of the hall element. What is described in Patent Document 1 is that the switch controller controls the switch to switch the directions of currents at opposite two terminals of the four terminals, so that the polarity of the hall offset voltage specific to the hall element having four terminals alternates four times per cycle. The differential amplifier amplifies the voltage of the opposite two terminals in a direction orthogonal to the direction of the current. In the switch operation of the switch for switching the direction of the current, if the switch operation makes the direction of the current opposite to the previous one, the switch controller controls the switch in synchronization therewith to reverse the polarity of the voltage from the differential amplifier. In addition, in synchronization with the immediately following switch operation of switching the direction of the current, the switch controller controls the switch to reverse the polarity of the voltage from the differential amplifier.
Further, for example, Patent Document 2 is related to a magnetism detection device enabling to have high-speed responsiveness, while excluding disturbance noises received by the hall element or the amplifier that amplifies the signal from the hall element.
Moreover, for example, Patent Document 3 is related to a magnetism detection device capable of removing the offset voltage of the magnetism detection sensor and the offset voltage of the analog element in the analog circuit, and detecting the magnetism of the magnetism detection sensor with high accuracy.
FIG. 1 is a circuit configuration view illustrative of a conventional magnetism detection device. In FIG. 1, reference numeral 1 indicates a hall sensor, reference numeral 2 indicates a bias current generating circuit, reference numeral 3 indicates a first switch circuit, reference numeral 4 indicates a second switch circuit, reference numeral 5 indicates a differential amplifier, reference numeral 6 indicates a third switch circuit, reference numeral 7 indicates an integrator, and reference numeral 8 indicates a switch control circuit.
The conventional magnetism detection device illustrated in FIG. 1 is provided with: the hall sensor 1 that detects magnetism; the bias current generating circuit 2 that drives the hall sensor 1; the first switch circuit 3 that switches the direction of the bias current applied to the hall sensor 1; the second switch circuit 4 that switches the direction of the differential voltage corresponding to the magnetism detected by the hall sensor 1; the differential amplifier 5 that amplifies an output differential voltage V1 of the second switch circuit 4; the third switch circuit 6 that switches the polarity of the output differential voltage of the differential amplifier 5; the integrator 7 that integrates and amplifies an output differential voltage V2 of the third switch circuit 6; and the switch control circuit 8 that controls a switch timing of the first to third switch circuits.
By controlling the first switch circuit 3 and the second switch circuit 4, the bias current is supplied between opposite two terminals of four terminals of the hall sensor 1, whereas the remaining opposite two terminals of the four terminals are connected to the differential amplifier 5.
FIG. 2A to FIG. 2D are views illustrative of connection states of the hall sensor in respective chopper phases switched by the switch circuits illustrated in FIG. 1. As illustrated in FIG. 2A to FIG. 2D, four phases of 0°→90°→180°→270° are periodically changed in this order and connected. Expressions (1) to (4) represent a hall electromotive force V1H corresponding to the bias current and the direction and magnitude of the detected magnetic field, and the output differential voltage V1 of the hall sensor 1 in which an offset voltage V10 specific to the hall sensor 1 is added.
Herein, the output differential voltage V1 of the hall sensor 1 is defined by Vhp-Vhn.
                              0          ⁢          °          ⁢                      :                    ⁢                                          ⁢                                    V              1                        ⁡                          (                              t                =                                  n                  ×                                      T                    4                                                              )                                      =                              +                          V                              1                ⁢                H                                              +                      V                          1              ⁢              O                                                          (        1        )                                          90          ⁢          °          ⁢                      :                    ⁢                                          ⁢                                    V              1                        ⁡                          (                              t                =                                  n                  ×                                                            2                      ⁢                      T                                        4                                                              )                                      =                              +                          V                              1                ⁢                H                                              -                      V                          1              ⁢              O                                                          (        2        )                                          180          ⁢          °          ⁢                      :                    ⁢                                          ⁢                                    V              1                        ⁡                          (                              t                =                                  n                  ×                                                            3                      ⁢                      T                                        4                                                              )                                      =                              -                          V                              1                ⁢                H                                              -                      V                          1              ⁢              O                                                          (        3        )                                          270          ⁢          °          ⁢                      :                    ⁢                                          ⁢                                    V              1                        ⁡                          (                              t                =                                  n                  ×                                                            4                      ⁢                      T                                        4                                                              )                                      =                              -                          V                              1                ⁢                H                                              +                      V                          1              ⁢              O                                                          (        4        )            
The differential amplifier 5 amplifies the output differential voltage V1 of the second switch circuit 4 and the offset voltage V20 specific to the differential amplifier 5 at a fixed magnification A. By controlling the third switch circuit 6, the polarity of the output differential voltage of the differential amplifier 5 is periodically switched and supplied to the integrator 7. The output differential voltage V2 of the third switch circuit 6 is represented by expressions (5) to (8).
                              0          ⁢          °          ⁢                      :                    ⁢                                          ⁢                                    V              2                        ⁡                          (                              t                =                                  n                  ×                                      T                    4                                                              )                                      =                  A          ×                      (                                          +                                  V                                      1                    ⁢                    H                                                              +                              V                                  1                  ⁢                  O                                            +                              V                                  2                  ⁢                  O                                                      )                                              (        5        )                                          90          ⁢          °          ⁢                      :                    ⁢                                          ⁢                                    V              2                        ⁡                          (                              t                =                                  n                  ×                                                            2                      ⁢                      T                                        4                                                              )                                      =                  A          ×                      (                                          +                                  V                                      1                    ⁢                    H                                                              -                              V                                  1                  ⁢                  O                                            +                              V                                  2                  ⁢                  O                                                      )                                              (        6        )                                          180          ⁢          °          ⁢                      :                    ⁢                                          ⁢                                    V              2                        ⁡                          (                              t                =                                  n                  ×                                                            3                      ⁢                      T                                        4                                                              )                                      =                              -            A                    ×                      (                                          -                                  V                                      1                    ⁢                    H                                                              -                              V                                  1                  ⁢                  O                                            +                              V                                  2                  ⁢                  O                                                      )                                              (        7        )                                          270          ⁢          °          ⁢                      :                    ⁢                                          ⁢                                    V              2                        ⁡                          (                              t                =                                  n                  ×                                                            4                      ⁢                      T                                        4                                                              )                                      =                              -            A                    ×                      (                                          -                                  V                                      1                    ⁢                    H                                                              -                              V                                  1                  ⁢                  O                                            +                              V                                  2                  ⁢                  O                                                      )                                              (        8        )            
FIG. 3 is a view illustrative of the polarities of the output voltages in the respective chopper phases switched by the switch circuits illustrated in FIG. 1. FIG. 3 indicates polarities of the hall electromotive force V1H, the offset voltage V10 of the hall sensor 1, and the offset voltage V20 of the differential amplifier 5, and in addition, indicates polarities of the hall electromotive force V1H′, the offset voltage V10′ of the hall sensor 1, and the offset voltage V20′ of the differential amplifier 5, which are demodulated by the first switch circuit 3. While the chopper operations are being repeated “n” times from 0° to 270°, the integrator 7 integrates and amplifies the output differential voltage V2 of the third switch circuit 6.
FIG. 4 is a view illustrative of output waveforms of the integrator of FIG. 1. An output V3 of the integrator 7 is represented by Expression (9) and has a voltage waveform as illustrated in FIG. 4.
                              V          3                =                                            ∑                              t                =                0                                            t                =                nT                                      ⁢                                          V                2                            ⁡                              (                t                )                                              =                                    n              ×                              (                                                                            ∑                                              t                        =                        0                                                                    t                        =                                                  T                          4                                                                                      ⁢                                                                  V                        2                                            ⁡                                              (                        t                        )                                                                              +                                                            ∑                                              t                        =                                                  T                          4                                                                                            t                        =                                                                              2                            ⁢                            T                                                    4                                                                                      ⁢                                                                  V                        2                                            ⁡                                              (                        t                        )                                                                              +                                                            ∑                                              t                        =                                                                              2                            ⁢                            T                                                    4                                                                                            t                        =                                                                              3                            ⁢                            T                                                    4                                                                                      ⁢                                                                  V                        2                                            ⁡                                              (                        t                        )                                                                              +                                                            ∑                                              t                        =                                                                              3                            ⁢                            T                                                    4                                                                                            t                        =                                                                              4                            ⁢                            T                                                    4                                                                                      ⁢                                                                  V                        2                                            ⁡                                              (                        t                        )                                                                                            )                                      =                          n              ×              4              ⁢                              AV                                  1                  ⁢                  H                                                                                        (        9        )            
Herein, V1H_INT is an integrated and amplified waveform of V1H component in the output V3 of the integrator 7. V10_INT is an integrated and amplified waveform of V10 component in the output V3 of the integrator 7. V20_INT is an integrated and amplified waveform of V20 component in the output V3 of the integrator 7. The chopper modulation is done by the first switch circuit 3 to the third switch circuit 6, and therefore, the offset voltage V10 of the hall sensor 1 and the offset voltage V20 of the differential amplifier 5 are cancelled in each of the chopper operations of 0° to 270°. Only the hall electromotive force V1H can be integrated and amplified.
FIG. 5 is a view illustrative of frequency characteristics of a chopper modulation transfer function with respect to the hall sensor noise. The chopper modulation is done by the first switch circuit 3 to the third switch circuit 6, and therefore, the element noises of the hall sensor 1 and the differential amplifier 5 can be modulated to higher frequencies near the frequency band of an odd multiple of the chopper modulation frequency FCHP (=1/T). The noises can be suppressed by the characteristics of the integrator 7 at the later stage.